Dishwashing machines employ a variety of different types of dishwashing detergents including liquids, powders, solid cakes of detergent and paste detergents. For example, a solid detergent is disclosed in Fernholz U.S. Pat. No. 4,469,780 and '781. Slurried dishwashing detergents are disclosed, for example, in Sabatelli U.S. Pat. No. 4,147,650, Pruhs U.S. Pat. No. 4,511,487 and Ulrich U.S. Pat. No. 4,431,559. Solid and slurried dishwashing detergents are disclosed in Bruegge U.S. Pat. No. 4,681,696.
These dishwashing detergents can include a variety of different detergent compositions. For example, the detergents disclosed in the Fernholz '780 and '781 references are high caustic detergents including close to 50% sodium hydroxide. On the other hand, the slurry dishwashing detergent disclosed in Sabatelli as well as Pruhs and Ulrich are lower caustic detergents.
All of these dishwashing detergents include hardness sequestering systems. These are chemicals which would in some way bind calcium or magnesium to prevent the deposition of the magnesium or calcium salts on the dishes being washed. There are a variety of different ones. For the prevention of calcium hardness, it is important to include a polyphosphate, generally an alkali tripolyphosphate such as sodium tripolyphosphate.
There are many different types of phosphates used to sequester hardness ions in addition to the polyphosphates, as an example tetra alkali pyrophosphate such as tetra sodium pyrophosphate. The pyrophosphates are employed preferably to combat magnesium hardness. These accomplish a different result than the polyphosphates. They are frequently combined with the polyphosphates. For example, Sabatelli U.S. Pat. No. 3,535,258 discloses a combination of pyrophosphate with tripolyphosphate in a detergent composition. However, in the preferred formulation, as well as all specific embodiments disclosed therein, only low caustic detergents are disclosed.
A variety of different phosphates are discussed in the article "Monsanto Phosphates for Industry" as well as "Polyphosphate Detergents in Mechanical Dishwashing Solubilizing Action of Polyphosphates on Certain Metals."
A problem with liquid, paste and solid (i.e., nonpowdered) detergents is reversion of the polyphosphate. Reversion is a chemical reaction of the polyphosphate to form an orthophosphate which does not effectively sequester calcium ions and reacts with calcium to form salts which deposit on the dishes. In effect, the polyphosphate is rendered useless.
This problem of reversion is not a problem with powdered detergents. If these are stored under dry conditions, there is relatively little reversion that could possibly occur. With nonpowder detergents, however, reversion is a substantial problem. The Fernholz reference discloses one method of addressing reversion, i.e., formation of a caked detergent. This apparently provides some limited benefit with respect to reversion.
The greatest problem is encountered with high caustic detergents where the detergent is stored for a long period of time. Reversion does not become a critical problem until the caustic level reaches about 25 to 35%. At high caustic levels, i.e., 40%, reversion is a critical problem. Solid paste detergents also exhibit this concern.
One additional problem encountered with paste detergents, particularly highly loaded high caustic paste detergents, is formation of a solid. These can be dispensed in a variety of different ways and in some applications it is necessary that they remain in a paste form which can flow to a certain extent. Permitting these detergents to solidify could prevent their use with certain dispensers. However, the heat generated during the mixing step tends to cause hydration which in turn causes the formation of a solid.